Infrared reflective color pigment

ABSTRACT

The present invention provides new solid solutions having a corundum-hematite crystalline structure which are useful as inorganic color pigments. Solid solutions according to the present invention include a host component having a corundum-hematite crystalline structure which contains as guest components one or more elements from the group consisting of aluminum, antimony, bismuth, boron, chrome, cobalt, gallium, indium, iron, lanthanum, lithium, magnesium, manganese, molybdenum, neodymium, nickel, niobium, silicon, tin, titanium, vanadium, and zinc. Solid solutions according to the present invention are formed by thoroughly mixing compounds, usually metal oxides or precursors thereof, which contain the host and guest components and then calcining the compounds to form the solid solutions having the corundum-hematite crystalline structure. Some of the new solid solutions according to the present invention exhibit relatively low Y CIE tri-stimulus values and relatively high near infrared reflectance.

FIELD OF INVENTION

The present invention concerns new solid solutions which are useful as inorganic color pigments. More particularly, the present invention concerns new solid solutions having a corundum-hematite crystalline structure which are useful as inorganic color pigments, some of which exhibit low Y CIE tri-stimulus values and high reflectivity in the near infrared portion of the electromagnetic spectrum.

BACKGROUND

Chromium green-black hematite (basic chemical formula: Cr₂O₃) is an inorganic color pigment, C.I. Pigment Green 17, having a corundum-hematite crystalline structure. It is commonly used to impart a green color to ceramics, paints, polymers, and other materials. The DCMA Classification and Chemical Description of the Complex Inorganic Color Pigments, Third Addition (1991), published by the Dry Color Manufacturer's Association, states that its composition may include any one or a combination of the modifiers Al₂O₃ (alumina), Fe₂O₃ (iron oxide), or Mn₂O₃ (manganese oxide).

Chromium green-black hematite is one of the principle pigments used in the manufacture of green shade military camouflage paint and netting. In such applications, chromium green-black hematite is combined with cobalt bearing mixed metal oxides, such as cobalt containing spinel pigment V12600 available from Ferro Corporation of Cleveland, Ohio. This combination of pigments is effective in simulating the reflectivity of chlorophyl in the visible portion of the electromagnetic spectrum, being that portion of the spectrum which is viewable by the naked eye with wavelengths ranging from approximately 0.40 μm to 0.70 μm.

Chlorophyl, which is an organic pigment, generally exhibits a relatively uniform high degree of reflectivity in the near infrared, being that portion of the electromagnetic spectrum with wavelengths ranging from approximately 0.7 μm, to 2.5 μm. Cobalt, however, exhibits a strong absorption band (i.e., low reflectivity) in a portion ofthe near infrared with wavelengths ranging from approximately 1.2 μm to 1.6 μm. In recent years, advancements in imaging technology have made it possible to contrast known military green shade camouflage painted or covered objects from the background foliage in that portion of the near infrared. A substitute military green shade camouflage pigment which contains no cobalt and which closely matches the reflectivity of chlorophyl in the visible and near infrared is therefore highly desired.

In order to satisfy military specifications, a substitute green shade camouflage pigment would have to exhibit a dark drab green appearance in the visible portion of the spectrum and would also have to simulate the reflectance curve for chlorophyl in the near infrared. Generally speaking, known inorganic pigments which exhibit a low degree of reflectivity in the visible portion of the light spectrum (i.e., dark drab colored pigments) also tend to exhibit a correspondingly low degree of reflectivity (i.e., high absorption) in other portions of the light spectrum, including the near infrared. A chromium green-black hematite pigment manufactured by Bayer Corporation of Germany, product number AC 5303, was observed to exhibit a higher near infrared reflectance than other chromium green-black hematite sources (this pigment, however, does not have the desired dark drab appearance in the visible spectrum required for military green shade camouflage paint applications). It was found by chemical analysis that this pigment contained both alumina and titania (basic chemical formula: TiO₂) as minor additives. A search failed to disclose any references teaching the use of alumina and titania to improve the near infrared reflectance of chromium green-black hematite pigments.

SUMMARY OF THE INVENTION

The present invention provides new solid solutions having a corundum-hematite crystalline structure which are useful as inorganic color pigments. Solid solutions according to the present invention are comprised of a host component having a corundum-hematite crystalline structure which contains as a guest component one or more elements from the group consisting of aluminum, antimony, bismuth, boron, chrome, cobalt, gallium, indium, iron, lanthanum, lithium, magnesium, manganese, molybdenum, neodymium, nickel, niobium, silicon, tin, titanium, vanadium, and zinc. Solid solutions according to the present invention are formed by thoroughly mixing compounds, usually metal oxides or precursors thereof, which contain the host and guest components and then calcining the compounds to form the solid solutions having the corundum-hematite crystalline structure.

Some of the new solid solutions according to the present invention, such as for example chrome oxide as a host component containing the elements iron, boron, and titanium as guest components, exhibit dark drab colors in the visible and high reflectivity in the near infrared portions of the electromagnetic spectrum. One of the primary uses for new solid solutions having these properties would be as inorganic color pigments in military camouflage paint or netting applications, which would permit the radiation signature of a painted or covered object to be tailored to match the reflectance curve of the background foliage in the visible and near infrared portions of the electromagnetic spectrum. Because many of these new solid solutions exhibit relatively high near infrared reflectance, they would also be suitable for use in the general paint and polymer markets, most specifically for architectural applications where increased near infrared reflectance would result in lower heat build-up and thus lower energy costs.

The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments ofthe invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing percent reflectance at wavelengths ranging from 0.6 μm to 2.5 μm for the infrared reflective color pigments embodied in Examples 1, 6, and 7 herein as compared to a standard chromium green-black hematite pigment.

FIG. 2 is a graph showing percent reflectance at wavelengths ranging from 0.6 μm to 2.5 μm for the infrared reflective color pigments embodied in Examples 3, 4, and 5 herein as compared to a standard chromium green-black hematite pigment.

FIG. 3 is a graph showing percent reflectance at wavelengths ranging from 0.6 μm to 2.5 μm for the infrared reflective color pigments embodied in Examples 8, 10, and 11 herein as compared to a standard chromium green-black hematite pigment.

FIG. 4 is a graph showing Y CIE tri-stimulus values and percent reflectance as measured at a wavelength of 2.5 μm for the infrared reflective color pigments embodied in Examples 1 through 16 herein as compared to a standard chromium green-black hematite pigment.

DETAILED DESCRIPTION

In accordance with the present invention, solid solutions having a corundum-hematite crystalline structure can be obtained by using metal oxides, or precursors thereof, which form corundum-hematite lattices as host components and incorporating into them as guest components such metal oxides, or precursors thereof, the cations of which possess radii which are comparable with the radii of the metal ions, or the statistical mean of the radii of the metal ions of the host components. The guest components are incorporated in such proportions relative to one another that the ratio of the sum of the added cations to the sum of the added anions, while preserving statistical electro-neutrality in the lattice, is substantially 2:3. The total proportion of the guest components may be of any value, but the amount is preferably not greater than approximately the total amount of the host components. Conceptually, these additions can be considered in a manner similar to the Hund crystalline solid solution patents, U.S. Pat. Nos. 3,022,186, 3,091,544, and 4,285,726, the specifications of which are hereby incorporated by reference. Essentially, guest components or dopants can be added to the host component or crystalline lattice as long as the crystalline structure, charge, and cation sites of the host component are substantially conserved.

Table 1 below sets forth some of the various compounds which have a corundum-hematite crystalline structure and comparable lattice constant ranges so as to be capable of forming solid solutions according to the present invention:

TABLE 1 Representative Corundum-Hematite Crystalline Lattice Constants Formula A C JCPDS Card Number Cr₂O₃ 4.958Å 13.593Å  38-1479 Al₂O₃ 4.758Å 12.991Å 10-173 V₂O₃ 4.920Å 13.883Å 34-187 Ga₂O₃ 4.979Å 13.429Å  6-503 Fe₂O₃ 5.036Å 13.749Å 33-664 Mn₂O₃ 5.04Å 14.12Å 33-900 Ti₂O₃ 5.139Å 13.659Å 10-63  In₂O₃ 5.49Å 14.52Å 22-336 TiBO₃ 4.67Å 14.90Å 17-310 NiTiO₃ 5.032Å 13.791Å 33-961 MgTiO₃ 5.054Å 13.898Å  6-494 CoTiO₃ 5.068Å 13.922Å 15-866 ZnTiO₃ 5.078Å 13.927Å  25-1500 FeTiO₃ 5.088Å 14.093Å 29-733 MnTiO₃ 5.140Å 14.290Å 29-902 CrBO₃ 4.578Å 14.260Å 15-663 NiCrO₃ 4.925Å 13.504Å 22-748 FeBO₃ 4.613Å 14.470Å 21-423 FeMoO₃ 4.67Å 15.03Å 19-611 FeSn(BO₃)₂ 4.733Å 15.08Å 18-657 BiFeO₃ 5.57Å 13.84Å 20-169 AlBO₃ 4.463Å 13.746Å 32-4  Mg₃Al₂Si₃O₁₂ 4.755Å 13.360Å 30-788 NdAlO₃ 5.326Å 12.920Å 29-57  LaAlO₃ 5.344Å 13.110Å 31-22  MnSnO₃ 5.358Å 14.505Å 33-913 LiNbO₃ 5.149Å 13.862Å 20-631 LaCoO₃ 5.441Å 13.088Å  25-1060 MgSiO₃ 4.725Å 13.563Å 39-300 ZnSiO₃ 4.746Å 13.755Å 34-575 Mn(Sb,Fe)O₃ 5.226Å 14.325Å 20-699

It should be noted that Table 1 does not contain all of the possible compounds that can form solid solutions according to the present invention. For example, molybdenum can be incorporated as a guest component into a chrome oxide host according to the present invention. One would reasonably expect that since iron and chrome have lattice constants which are very close, and both form extensive solid solutions, CrMoO₃ would exist and be similar in crystal structure to FeMoO₃, although CrMoO₃ is not shown in Table 1. Solid solutions according to the present invention, irrespective of whether shown on Table 1, are comprised of a host component having a corundum-hematite crystalline structure which contain as a guest component one or more elements from the group consisting of aluminum, antimony, bismuth, boron, chrome, cobalt, gallium, indium, iron, lanthanum, lithium, magnesium, manganese, molybdenum, neodymium, nickel, niobium, silicon, tin, titanium, vanadium, and zinc.

Solid solutions according to the present invention are prepared by thoroughly dry or wet mixing compounds containing the host and guest components and then calcining the components to form a solid solution. Generally, the host and guest components will both comprise metal oxides. However, precursors thereof, meaning compounds which at the calcining temperature are converted in the presence of an oxidizing atmosphere into metal oxides can also be used. Such precursors include, for example: aluminum hydrate (basic chemical formula: Al(OH)₃)) which can be used as a precursor for aluminum oxide; manganese carbonate (basic chemical formula: MnCO₃) which can be used as a precursor for manganese sesquioxide (basic chemical formula: Mn₂O₃); and indium hydroxide (basic chemical formula: In(OH)₃) which can be used as a precursor for indium oxide (basic chemical formula: In₂O₃). It will be appreciated that a wide variety of precursors can be used to practice the invention, and that the selection of precursors is not per se critical.

The host and guest components are preferably milled to a fine powder and then thoroughly mixed in the appropriate proportions. Milling fineness is not critical, but average particle size diameters from approximately 0.2 μm to about 5 μm are preferred. Standard pigment grade metal oxide powders and precursors currently available on the market can generally be used without additional milling prior to mixing. Mixing can occur in a dry state, or the powders can be mixed in solution, dried, and then milled again if necessary to break up any particle agglomerations. The mixture is then calcined to form crystalline solid solutions. Calcination can be achieved in a standard refractory sagger, kiln, or other suitable device to form the solid solutions. Calcination can also be achieved through the use of a rotary calciner. Calcination sufficient to form solid solutions generally occurs at a temperature of from about 1,750° F. to about 2,250° F. for about 0.5 to about 24 hours. However, it will be understood to those practicing the invention that a wide range of calcination temperatures and times can be used, and that so long as the temperature and length of time used is sufficient to form the solid solutions according to the present invention, the temperature and length of time of calcination is not per se critical.

In most instances, it is unnecessary to mill or grind solid solutions formed according to the present invention after calcination. However, when a smaller average particle size is desired or when a particular application requires a strict avoidance of minor particle agglomerations, additional milling or grinding of the solid solutions can be performed.

It should be apparent to those skilled in the art that the method of the present invention can be practiced to manufacture a wide variety of solid solutions having utility as inorganic pigments for use in ceramics, polymers, paints, and other materials. The selection of guest and host components used to form solid solutions can readily be made within the total specification disclosure. In similar fashion, it is to be appreciated that the process steps ofthe method, including mixing, calcination, and product recovery, are generally conventional and thus can be readily determined by those skilled in the art.

The following examples are intended to illustrate the invention without limiting it in any way. All raw materials referenced in the examples are standard pigment grade powders unless otherwise indicated.

EXAMPLE 1

93.46 grams chrome oxide, 0.94 grams iron oxide, 2.38 grams aluminum oxide from aluminum hydrate and 1.88 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 2

93.02 grams chrome oxide, 0.93 grams iron oxide, 3.35 grams aluminum oxide from aluminum hydrate and 0.83 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 3

93.23 grams chrome oxide, 0.92 grams iron oxide, 0.43 grams molybdenum trioxide (basic chemical formula: MoO₃), 3.11 grams aluminum oxide from aluminum hydrate and 0.57 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 4

95.49 grams chrome oxide, 0.96 grams iron oxide, 1.63 grams boron oxide (basic chemical formula: Bi₂O₃) and 1.92 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 5

86.79 grams chrome oxide, 0.96 grams iron oxide, 10.59 grams bismuth trioxide (basic chemical formula: Bi₂O₃) and 1.75 grams titanium dioxide were thoroughly mixed in awaring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 6

91.86 grams chrome oxide, 0.92 grams iron oxide, 2.42 grams aluminum oxide from aluminum hydrate and 3.56 grams tin oxide (basic chemical formula: SnO) were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 7

93.87 grams chrome oxide, 0.94 grams iron oxide, 2.39 grams aluminum oxide from aluminum hydrate and 1.45 grams silicon dioxide (basic chemical formula: SiO₂) were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 8

93.05 grams chrome oxide, 0.93 grams manganese sesquioxide from manganese carbonate, 2.68 grams aluminum oxide from aluminum hydrate and 1.39 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000 ° F. for three hours.

EXAMPLE 9

89.96 grams chrome oxide, 0.90 grams iron oxide, 6.13 grams indium oxide from indium hydroxide and 1.81 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 10

90.06 grams chrome oxide, 0.91 grams iron oxide, 7.22 grams lanthanum oxide (basic chemical formula: La2O₃) and 1.81 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 11

89.85 grams chrome oxide, 0.90 grams iron oxide, 7.44 grams neodymium oxide (basic chemical formula: Nd₂(SO₄)₃) and 1.81 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 12

95.08 grams chrome oxide, 0.96 grams iron oxide, 2.50 grams aluminum oxide from aluminum hydrate and 0.06 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 13

79.36 grams chrome oxide, 0.80 grams iron oxide, 9.98 grams aluminum oxide from aluminum hydrate and 4.27 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 14

99.06 grams chrome oxide and 0.94 grams iron oxide were thoroughly mixed in a Waring blender and calcined in a crucible at 2,000° F. for three hours.

EXAMPLE 15

93.15 grams chrome oxide, 0.88 grams iron oxide, 3.06 grams aluminum oxide from aluminum hydrate and 1.19 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 1,850° F. for three hours.

EXAMPLE 16

48.46 grams chrome oxide, 48.46 grams iron oxide, 1.58 grams aluminum oxide from aluminum hydrate and 0.61 grams titanium dioxide were thoroughly mixed in a Waring blender and calcined in a crucible at 1,850° F. for three hours.

FIG. 1 is a graph showing percent reflectance at wavelengths ranging from 0. 6 μm to 2.5 μm for infrared reflective color pigments prepared as described in Examples 1, 6, and 7 above as compared to a standard chromium green-black hematite pigment available as G-4099 from Elementis Pigments, Inc. of Fairview Heights, Ill. Percent reflectance measurements were made on neat pressed cup powder samples using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory. FIG. 1 shows that some of the infrared reflective color pigments according to the present invention exhibit significantly higher near infrared reflectance than known chromium green-black hematite pigments. FIG. 1 also shows that elements such as silicon and tin can be substituted for titanium in the crystal lattice structure of pigments according to the present invention.

FIG. 2 is a graph showing percent reflectance at wavelengths ranging from 0.6 μm to 2.5 μm for infrared reflective color pigments prepared as described in Examples 3, 4, and 5 above as compared a standard chromium green-black hematite pigment available as G-4099 from Elementis Pigments, Inc. of Fairview Heights, Ill. Percent reflectance measurements were made on neat pressed cup powder samples using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory. FIG. 2 shows that elements such as bismuth, boron, and molybdenum can be substituted for the aluminum cation in the crystal lattice structure of pigments according to the present invention, allowing for adjustments ofthe visible color while retaining the preferable high near infrared reflectance.

FIG. 3 is a graph showing percent reflectance at wavelengths ranging from 0.6 μm to 2.5 μm for infrared reflective color pigments prepared as described in Examples 8, 10, and 11 above as compared to a standard chromium green-black hematite pigment available as G-4099 from Elementis Pigments, Inc. of Fairview Heights, Ill. Percent reflectance measurements were made on neat pressed cup powder samples using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory. FIG. 3 shows that elements such as manganese, lanthanum, and neodymium can be substituted for the iron or aluminum cations in the crystal lattice structure of pigments according to the present invention, allowing for adjustments of the visible color while retaining the preferable high near infrared reflectance. By adjusting the quantity and identity of the combinations of cations in crystalline lattice structure in pigments according to the present invention, a wide range of colors can be produced.

FIG. 4 is a graph showing Y CIE tri-stimulus values versus percent reflectance measured at a wavelength of 2.5 μm for the infrared reflective color pigments prepared as described in Examples 1 through 16 above as compared to a standard chromium green-black hematite pigment available as G-4099 from Elementis Pigments, Inc. of Fairview Heights, Ill. Y CIE tri-stimulus values were measured on neat pressed cup powder samples using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, the text of which can be purchased from the American Society for Testing and Materials of West Conshohocken, Pa. Percent reflectance measurements were made on neat pressed cup powder samples using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory. FIG. 4 shows that some of the new solid solutions according to the present invention, such as those embodied in Examples 1, 3, 4, 8, 10, 11, 13, and 16, exhibit relatively low Y CIE tri-stimulus values (13.53 or less) and relatively high near infrared reflectance at 2.5 μm (71.7% or higher). Generally speaking, the Y CIE tri-stimulus value refers to the lightness or darkness of a color, with a lower Y CIE tri-stimulus value correlating with a darker color.

Table 2 below lists the Y CIE tri-stimulus values, x and y CIE chromaticity coordinates, and percent reflectance at various wavelengths in the near infrared spectrum for the solid solutions embodied in Examples 1 through 16 as compared to a standard chromium green-black hematite pigment available as G-4099 from Elementis Pigments, Inc. of Fairview Heights, Illinois. Y CIE tri-stimulus values and x and y CIE chromaticity coordinates were measured on neat pressed cup powder samples using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, the text of which can be purchased from the American Society for Testing and Materials of West Conshohocken, Pa. Percent reflectance measurements were made on neat pressed cup powder samples using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory.

TABLE 2 Color and % IR Reflectance (Pressed Cups) Example Y x y 0.72 μm 0.80 μm 0.90 μm 1.0 μm 1.5 μm 2.5 μm Standard 20.33 .315 .409 35.0 59.6 58.9 57.4 55.0 56.0 1 11.91 .328 .378 25.0 57.0 67.8 66.2 72.0 78.0 2 15.99 .331 .406 24.2 59.6 71.3 69.0 73.9 76.3 3 12.61 .333 .391 28.6 66.5 79.1 76.2 84.2 88.5 4 10.77 .323 .372 23.8 62.2 74.8 73.0 77.4 87.3 5 12.64 .332 .386 27.0 59.3 66.2 64.3 64.3 74.8 6 16.53 .326 .399 33.3 63.1 67.2 66.0 67.7 71.9 7 16.81 .326 .402 32.8 59.0 61.9 60.6 59.4 61.5 8 10.71 .321 .374 21.5 50.5 68.3 74.8 76.0 81.0 9 12.04 .331 .379 25.5 57.2 63.8 61.1 60.9 75.0 10 11.50 .330 .376 25.2 60.0 68.0 66.2 70.0 81.4 11 11.10 .326 .368 24.0 58.1 70.0 69.2 75.2 80.0 12 14.28 .330 .405 29.7 56.1 59.5 58.1 57.0 58.0 13 13.53 .334 .398 31.6 64.1 72.7 70.0 71.2 83.2 14 15.35 .334 .408 27.4 59.2 62.1 61.0 60.0 65.0 15 14.82 .330 .403 30.6 60.1 67.2 65.5 69.9 75.4 16 6.43 .328 .325 11.6 22.1 34.5 37.0 63.6 71.7

One of the primary uses for new solid solutions according to the present invention which exhibit relatively low Y CIE tri-stimulus values and relatively high near infrared reflectance would be as inorganic pigments in military camouflage paint or netting applications. By using these pigments in place of known pigments which contain cobalt, the radiation signature of a camouflage painted or covered object can be tailored to match the background in the visible and near infrared portions of the electromagnetic spectrum. Moreover, because many of these new solid solutions exhibit relatively high near infrared reflectance in a range of colors, they would also be suitable for use in the general paint and polymer markets, most specifically for architectural applications, such as vinyl siding, where increased near infrared reflectance would result in lower heat build-up and thus lower energy costs.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and illustrative examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A solid solution comprising a host component having the corundum-hematite crystalline structure, said host component comprising chrome oxide, and a guest component, said guest component being incorporated into the crystalline lattice structure of said host component, said guest component comprising metal oxides or precursors thereof having as cations the elements iron, aluminum, and titanium, said solid solution having a Y CIE tri-stimulus value below 14 as measured on a neat pressed cup powder sample using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, and a reflectance above 70% as measured on a neat pressed cup powder sample using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory at a wavelength of 2.5 μm.
 2. The solid solution as in claim 1 wherein cations ofthe element chrome comprise from about 45% to about 98% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.1% to about 53% of the weight of all cations in the crystalline lattice, cations of the element aluminum comprises from about 0.5% to about 15% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice.
 3. The solid solution as in claim 1 wherein cations ofthe element chrome comprise from about 90% to about 98% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.25% to about 2% of the weight of all cations in the crystalline lattice, cations of the element aluminum comprises from about 1.5% to about 4% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 1% to about 3% of the weight of all cations in the crystalline lattice.
 4. The solid solution as in claim 1 wherein cations ofthe element chrome comprise from about 70% to about 90% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.25% to about 1.5% of the weight of all cations in the crystalline lattice, cations of the element aluminum comprises from about 2.5% to about 17.5% of the weight of all cations in the crystalline lattice, and cations ofthe element titanium comprise from about 2% to about 7% of the weight of all cations in the crystalline lattice.
 5. The solid solution as in claim 1 wherein cations ofthe element chrome comprise from about 45% to about 60% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 40% to about 55% of the weight of all cations in the crystalline lattice, cations of the element aluminum comprises from about 0.25% to about 4% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 0.1% to about 3% of the weight of all cations in the crystalline lattice.
 6. A solid solution comprising a host component having the corundum-hematite crystalline structure, said host component comprising chrome oxide, and a guest component, said guest component being incorporated into the crystalline lattice structure of said host component, said guest component comprising metal oxides or precursors thereof having as cations the elements iron, aluminum, titanium, and molybdenum, said solid solution having a Y CIE tri-stimulus value below 14 as measured on a neat pressed cup powder sample using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, and a reflectance above 70% as measured on a neat pressed cup powder sample using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory at a wavelength of 2.5 μm.
 7. The solid solution as in claim 6 wherein cations of the element chrome comprise from about 80% to about 98% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, cations of the element aluminum comprises from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, cations of the element titanium comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, and cations of the element molybdenum comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice.
 8. A solid solution comprising a host component having the corundum-hematite crystalline structure, said host component comprising chrome oxide, and a quest component, said quest component being incorporated into the crystalline lattice structure of said host component, said quest component comprising metal oxides or precursors thereof having as cations the elements iron, boron, and titanium, said solid solution having a Y CIE tri-stimulus value below 14 as measured on a neat pressed cup powder sample using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, and a reflectance above 70% as measured on a neat pressed cup powder sample using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory at a wavelength of 2.5 μm.
 9. The solid solution as in claim 8 herein cations of the element chrome comprise from about 80% to about 98% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, cations of the element boron comprises from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice.
 10. A solid solution comprising a host component having the corundum-hematite crystalline structure, said host component comprising chrome oxide, and a guest component, said quest component being incorporated into the crystalline lattice structure of said host component, said quest component comprising metal oxides or precursors thereof having as cations the elements iron, lanthanum, and titanium, said solid solution having a Y CIE tri-stimulus value below 14 as measured on a neat pressed cup powder sample using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, and a reflectance above 70% as measured on a neat pressed cup powder sample using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory at a wavelength of 2.5 μm.
 11. The solid solution as in claim 10 wherein cations of the element chrome comprise from about 80% to about 98% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, cations of the element lanthanum comprises from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice.
 12. A solid solution comprising a host component having the corundum-hematite crystalline structure, said host component comprising chrome oxide, and a quest component, said guest component being incorporated into the crystalline lattice structure of said host component, said guest component comprising metal oxides or precursors thereof having as cations the elements iron, neodymium, and titanium, said solid solution having a Y CIE tri-stimulus value below 14 as measured on a neat pressed cup powder sample using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, and a reflectance above 70% as measured on a neat pressed cup powder sample using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory at a wavelength of 2.5 μm.
 13. The solid solution as in claim 12 wherein cations of the element chrome comprise from about 80% to about 98% of the weight of all cations in the crystalline lattice, cations of the element iron comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, cations of the element neodymium comprises from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice.
 14. A solid solution comprising a host component having the corundum-hematite crystalline structure, said host component comprising chrome oxide, and a guest component, said guest component being incorporated into the crystalline lattice structure of said host component, said guest component comprising metal oxides or precursors thereof having as cations the elements manganese, aluminum, and titanium, said solid solution having a Y CIE tri-stimulus value below 14 as measured on a neat pressed cup Powder sample using a Diano Matchscan II Spectrophotometer using a 1931 CIE 2 degree observer with a C Illuminant in accordance with the procedures set forth in ASTM Standard E308 (1996), Standard Practice for Computing the Colors of Objects by Using the CIE System, and a reflectance above 70% as measured on a neat pressed cup powder sample using a Perkin Lambda 19 Spectrophotometer equipped with a Labsphere RSA-PE-19 reflectance spectroscopy accessory at a wavelength of 2.5 μm.
 15. The solid solution as in claim 14 wherein cations ofthe element chrome comprise from about 80% to about 98% of the weight of all cations in the crystalline lattice, cations of the element manganese comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, cations of the element aluminum comprises from about 0.1% to about 10% of the weight of all cations in the crystalline lattice, and cations of the element titanium comprise from about 0.1% to about 10% of the weight of all cations in the crystalline lattice. 